docs: def subtract(self, val):
          """
          Subtract a constant value or an image from all images.
  
          Parameters
          ----------
          val : int, float, or ndarray
              Value to subtract.
          """
          if isinstance(val, ndarray):
              if val.shape != self.value_shape:
                  raise Exception('Cannot subtract image with dimensions %s '
                                  'from images with dimension %s' % (str(val.shape), str(self.value_shape)))
  
          return self.map(lambda x: x - val, value_shape=self.value_shape)